1. Field of the Invention
This invention pertains generally to in situ formation of reactive barriers in soil, and more specifically the invention relates to in situ formation of magnetite (Fe3O4) reactive barriers created by mixing solutions injected into soil. The solutions react, fill void spaces and coat particles in soil with reactive magnetite thereby resulting in a very large reactive surface area that allows water to pass, but sequesters or destroys radionuclides, heavy metals or organics.
2. Description of the Related Art
Magnetite and goethite have been demonstrated to provide an effective barrier for preventing the transport of certain radioactive minerals including plutonium and heavy metals. Techniques (Boyd, et al., 1985) have been developed to produce goethite and magnetite from water soluble compounds, and involve, for example, reacting a soluble Fe(II) salt solution with a base to form Fe(III)/Fe(II) solid. The resulting magnetite is extremely stable both kinetically and thermodynamically. Moreover, for purposes of hazardous material remediation, magnetite is one of the only minerals that has been demonstrated to form an effective reactive barrier for transport of the heavy metal technetium, Tc. It also is effective in creating a barrier to the passage of actinides, iodine, heavy metals and some organic substances.
Leakage of radioactive materials, heavy metals, and other contaminants from storage tanks and subsequent migration of those substances away from the containment area continues to be a significant unresolved problem at a number of government and private storage sites. Various techniques are used to try to isolate leaking storage containers and contaminated soil to prevent movement of contaminants into uncontaminated soil and especially groundwater. Tc leakage from tanks at the Hanford site in Washington state, for example, is especially of major concern, in part because of the tendency of Tc to form highly water-soluble compounds in its higher oxidation states. Those water-soluble compounds can exhibit very high mobility in soil.
One approach to remediating soil contamination is simply to attempt to dig up and remove soil containing hazardous material. This, however, is costly, and disturbance of contaminated soil carries the risk that some contaminants will be missed or released and left to migrate further. Excavation also has a negative effect on soil stability. Excessive digging and excavation around waste tanks, for example, has the potential to aggravate waste transport by shaking loose corroded containment drums and already contaminated soil.
Another approach is to establish barriers in the soil of a contaminated site in order to prevent migration of contaminants beyond the barriers. Barriers of this sort that are in use at various sites around the United States and abroad include vertical sleeves placed in trenches surrounding a site. They also include walls formed through the injection of highly pressurized grout in holes drilled in the soil. Emplacement of such barriers typically requires greatly disturbing the soil and often there is no convenient way to create a xe2x80x9cfloorxe2x80x9d or continuous barrier beneath the leaking tank or contaminated region. Consequently, the sequestration of the contaminants is incomplete and contaminants can continue to migrate downward and perhaps outward, thereafter. For areas under waste tanks, waste trenches and certain geological formations, forming a continuous barrier is difficult and sometimes impossible.
Various efforts have been made to create reactive barriers which, in addition to forming a physical obstruction to the movement of contaminant substances, actively attract and chemically bind contaminants. Even more advantageous, however, would be formation of a permeable xe2x80x9cbarrierxe2x80x9d though which water can freely pass, but which immobilizes and sequesters pollutants. The need remains, therefore, for reactive barriers capable of being formed in situ in soil that are thermodynamically and kinetically stable, that serve to sequester radioactive materials, iodine, organic pollutants, and heavy metals including Tc while allowing water to pass though.
Therefore, it as an object of the present invention to provide reactive barriers capable of binding and inhibiting migration of soil contaminants including radioactive materials, organics, iodine and heavy metals including Tc.
It is another object of the invention to provide a method of in situ formation of reactive barriers in soil wherein those barriers include magnetite.
It is another object of the invention to provide barriers, formed in situ, wherein magnetite is dispersed throughout a. large volume of soil.
It is yet another object of the invention to utilize capillary action of soil, and/or movement of groundwater, and/or soil washing to help mix water-soluble reagents injected at low pressure to react and chemically form a reactive barrier in the soil, itself.
An advantage of the present invention is that by adequately saturating soil with necessary chemical reagents, the barrier formed using the methods described here can extend throughout a large volume of soil, for example, surrounding and including soil containing pollutants.
Another advantage of the present invention is that it accommodates use of lowpressure injection techniques, should minimal disturbance of the soil be needed or desired in forming the reactive barrier.
Another advantage of the invention is that the chemical processes involved are suited also to high-pressure injection of reagents, for circumstances wherein rapid injection is desired or required.
Another advantage of the present invention is that it is economical in comparison with traditional digging and excavation decontamination methods, and no backfilling is required.
These and other objects and advantages of the present invention are fulfilled and. satisfied by the claimed invention which involves introducing into soil an aqueous solution of iron salt and, separately, an aqueous basic solution, such as NaOH, KOH or NH3. Then, by way of in situ mixing, magnetite is formed in the soil thereby creating a reactive barrier that sorbs contaminants including actinides, heavy metals, iodine, technetium and organic materials. According to an embodiment of the invention, a two-step introduction of the above-mentioned solutions can be used. In the first step, according to one embodiment, roughly equivalent amounts and concentrations of iron salt solution and base solution are introduced into soil and allowed to mix, thereby forming Fe(III) oxide, iron oxyhydroxides and other similar compounds. According to the invention, these compounds serve to remove oxygen from the soil matrix. In the second step, iron salt solution and base solution are introduced into the same soil (for example, using similar concentrations), but this time using more base solution than iron salt solution (optimally, in a volume ratio of about 2:1, base to salt, using solutions of similar concentration). As these reagents react, magnetite is formed in situ in the soil. Various embodiments showing alternative approaches to introducing reagents into the soil are disclosed in this specification.
Additional advantages and novel features will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.